The use of a pre-etched window as a mask for laser drilling blind vias in a multi-layer circuit board is generally known. Dense pin count and/or dense component placement on multi-layer circuit boards or polymer based multi-chip modules can create an interconnect density problem referred to in the industry as “via starvation”. One solution to the problem of “via starvation” is the formation of blind vias interconnecting one or more layers in a multi-layer circuit board or multi-chip module.
One quality criterion in blind via formation is copper capture pad appearance. Capture pad appearance is sometimes referred to as “shiny”, “melted too much” or “dark”, and tends to be a very subjective criterion. Some customers specify the desired process parameter by this subjective measure in addition to throughput considerations. It has been observed that laser operating parameters affect the appearance of the copper capture pad. For high fluence process, the pad tends to be melted showing a “shiny” appearance. When the fluence is set very low, the appearance is somewhat “dark”. The number of pulses applied to the material can also affect the appearance. Another observation is that the appearance varies depending on the laser operating parameter or characteristics, such as pulse width. It has also been observed that the appearance varies depending on the capture pad geometry even when the same laser processing parameters are used. When a variation in quality is observed in the entire panel process, it is sometimes difficult to resolve the source of the quality variation, since data coming from the computer aided design (CAD) system relates only to the desired drilling location.
Ultraviolet (UV) laser and image projection processing has been used for drilling blind via in integrated circuits (IC) packaging substrates. The current process typically applies a single set of laser operating parameters to all vias or holes that are intended to yield the same geometry between two layers in a printed circuit design. However, due to varying capture pad geometry, the end results are sometimes less than desirable. When applying a fixed set of laser operating parameters, the results vary from “dark” copper when drilling in a solid copper plane, to delaminated copper on 110 micrometer (Mm) copper capture pads.